Anti-scatter grids are used in x-ray systems so that when an object under examination is illuminated with x-rays only direct information-bearing primary radiation reaches the detector from the x-ray tube. The scattered radiation which makes the image noisy is blocked by the anti-scatter grid in order to improve the quality of the x-ray image. Anti-scatter grids consist of very thin strips of lead which are embedded into paper, with up to 80 strips per centimeter being used.
These lead strips of the anti-scatter grid are not arranged in parallel but are turned at a slight angle to each other and aligned to a specific optimum focal distance between the x-ray tube and the detector, for example 1500 mm. In practice however x-ray systems are operated at different tube-detector distances so that they are defocused, producing shadowing in the edge area of the x-ray image recording. Shadowing can also arise because the tube is not centered, that is it is not located precisely over the center point of the anti-scatter grid. Heel effect refers to the reduction in the intensity of rays on the anode side through the x-ray tube itself. Shadowing can also occur because of an incorrectly positioned or defective grid. This shadowing normally increases towards the edge at right angles to the direction of the lead laminations, while remaining constant in the direction of the lead laminations.